The invention relates to a transformer for balancing currents, also referred to as a current balancing transformer.
Current balancing transformers are used for balancing alternating currents. The advantages of these passive components lie in their simplicity, since no active regulation is needed.
FIG. 1 shows a circuit diagram of a transformer 10 having a primary winding 12 and a secondary winding 14. In balancing the current, a transformer makes use of the fact that the ratio of the current IP in the primary winding and the current IS in the secondary winding is the inverse of the ratio of the number of windings in the primary winding NP to the number of windings in the secondary winding NS, as described in the equation below.
                                          I            s                                I            p                          =                              N            p                                N            s                                              [        1        ]            
Thus where NP is equal to NS, the current IS in the secondary winding also corresponds to the current IP in the primary winding. It is of course clear that if there is a difference in the winding ratio NP, NS in the primary and secondary winding, a difference in the current ratio between the two windings may also be achieved.
A backlight is a necessary requirement for LCD displays in order to achieve a visible image, since LCD displays themselves do not emit light. For this kind of backlight, cold cathode fluorescent lamps (CCFLs) are generally employed, these lamps being supplied with a high-frequency AC voltage of some 1000 volts at a current of 5 to 6 milliamperes. However, since several lamps are employed in the backlight, it is necessary to control the brightness of the lamps, making it possible to achieve a uniform illumination of the LCD display. The brightness of the lamps is controlled in that each lamp is supplied with the same operating current. For this purpose, an appropriate device is needed to uniformly distribute the current over the number of lamps, current balancing transformers being preferably employed.
FIG. 2 shows a schematic circuit diagram of this kind of backlight device having current balancing transformers 10a and 10b. Each primary winding of the transformers 10a and 10b is coupled in series to two cold cathode fluorescent lamps 20a and 22a or 20b and 22b respectively and connected to a high voltage source 24. The secondary windings of the transformers 10a and 10b are interconnected in series to a closed circuit. In this secondary circuit, the same current IS flows through both secondary windings of the transformers 10a and 10b, so that the same current IP also flows in the primary circuit of the two transformers, assuming the transformers are identical. The current balancing circuit shown in FIG. 2 can also be extended to include more than two transformers. However, the quality of current balancing using this kind of circuit is often unsatisfactory. The reason for this is that the transformers have a main inductance that in practice also has to be taken into account and that partly gives rise to large tolerances between the individual currents of the transformers.
FIG. 3 shows a circuit diagram of a transformer 10 comprising a primary winding 12, a secondary winding 14 and a main inductance 16 as depicted. The main inductance 16 generates an additional current IL on the primary side of the transformer that is also referred to as magnetization current. Due to a relatively large tolerance in the main inductance dL/L between the transformers, this current IL can have a tolerance of 20% between the individual transformers 10a, 10b. Based on the series connection of the secondary windings the secondary current is equal in all transformers but the primary currents in the transformers and therefore, the lamp currents differ in the main inductances (16) due to the tolerance. The formula below describes the influence of the tolerance of the main inductance on the change in the primary current:
                                          dI            p                                I            p                          =                                                            (                                                      I                    L                                                        I                    p                                                  )                            2                        ·                                          dI                L                                            I                L                                              =                                    -                                                (                                                            I                      L                                                              I                      p                                                        )                                2                                      ·                                          d                ⁢                                                                  ⁢                L                            L                                                          [        2        ]            
It can be seen that the smaller the magnetization current IL in relation to the primary current, the smaller is the change in the primary current dIS/IS. One way of achieving this is to make the main inductance sufficiently large by having, for example, a large number of windings of the primary or secondary windings respectively. In doing this, however, the size and power loss of the transformer is increased, along with manufacturing costs. WO 2005/038828, for example, suggests using a transformer having high permeability in order to reduce reactive current. However, cores having high relative permeability are again quite expensive.